A METHOD FOR SPECTRA-COLOR B-SCAN ULTRASONOGRAPHY

b y E. Holasek, BSEE; L. A. Gans, BA; E. W. Purnell, MD; and A. Sokollu, ScD

ABSTRACT A new technique which records color coded frequency information in B-scan ultrasonog- raphy is described. The frequency spectrum of a wide band transducer (6MHz-12MHz) is divided into three component bands each recorded as one of the three primary colors of light. The results of tissue model studies are presented.

Indexing Words Spectra-color ultrasonography Color ultrasonography

INTRODUCTION

Ocularultrasonograms are currently made with a single frequency band centered in the range of 5 MHz to 20 MHz. Additional informa- tion about tissues may be gained by analysis of their response to various frequencies of ultra- sound. To incorporate this additional informa- tion into B-scan ultrasonography, a method was devised to divide the signal spectrum from a wide band transducer into three separate compo- nent bands and record the response on a single color photograph with the primary colors of light representing the three component fre- quencies. We have termed this method of repre- sent ing frequency information by color, Spectra-Color Ultrasonograpliy (S.C.U.).

METHOD

The basic instrumentation for S.C.U. is a high resolution, digitally controlled, sector scanner designed for ophthalmic application. A full sector scan of 22.5 degrees is performed in 0.5 seconds. Each sector contains 400 lines of intensity modulated B-scan. The transducer assembly is mounted in a mechanical stage and coupled to a water bath which permits immer- sion of various experimental targets and physical models. For clinical use it is coupled to a special

From the Division of Ophthalmology. Department of Surgery,

Received July 9. 1 9 7 5 : revision accepted July 2 5 , 1975 . For reprints contact : Secretary, Laboratory for Research in

Ophthalmology, Wearn Research Bldg., 2065 Adelbert Road, Cleveland, Ohio 44106.

Case Western Reserve Universitv. Cleveland, Ohio 44106.

This work was supported by N.E . I . grant E Y 00224-14.

S.C.U. B-scan

pair of goggles which are worn by patients for scans of the eye and orbit.

A block diagram of the instrumentation is shown in Figure 1. The Transmitter sends a high energy pulse which excites the ultrasonic trans- ducer. The Transducer is wide band, with a center frequency of 9 MHz and a fractional bandwidth of 66 percent, covering the fre- quency range from 6 MHz to 12MHz. The ultrasound is focused by an acoustic lens with a focal length of 70 mm. Axial resolution is 0.3 mm in soft tissue. Lateral resolution varies from 0.4 mm to 1.0 mm depending on the signal strength. Eighty decibels of amplification is provided by two Amplif iers of 35 MHz band- width. The investigator uses a fixed Attenuator to select the maximum linear range to produce the optimum display pattern (see discussion). The amplified wide band radio frequency signal is detected to produce a video signal super- imposed onto a ramp voltage and applied to a cathode ray tube for intensity modulated dis- play. The Display has a five inch diameter and a spot size of approximately 0.1 mm. The Spectra- Color Ultrasonogram is taken by a Camera mounted in front of the cathode ray tube. The Camera is a Hasselblad, Model 500EL/M with motor driven film advance and electronic shutter control. The lens is a Ziess 80 mm Planar f2.8 with Compur shutter and Extension Tube No. 55.

For Spectra-Color Ultrasonography, a four- pole Butterworth Voltage Tuned Filter (Fig. 1) receives three successive DC voltages to generate

175 VOLUME 3. NUMBER 3

Y I D L BAND

AHPL I F I E R

I I

SUIICH

V l O E BAND

A M P L I F I E R TH

VOLTAGE

COLOR

(ONTROL

F IGURE 1. Block diagram of instrumentation for Spectra-Color Ultrasonography. See text for explanation.

the component frequency bands. Figure 2 shows the three component bands, designated low, middle, and high, having center frequencies of 7MHz, 9MHz, and l l M H z , and a 2MHz half-power bandwidth. These frequencies are arbitrarily assigned the three primary colors of light, red, green, and blue, respectively, by introducing light filters between the video Dis- play and the Camera (Fig. 1 ) . The filters are mounted in a light filter wheel which is rotated by a stepping Motor to sequentially interpose each of the filters in coordination with each of three successive sector scans. The filters are Kodak Wratten Gelatin, No. 25 (red), No. 57 (green), and No. 47 (blue).

The Color Control device (Fig. 1) opens the camera shutter, sequentially applies the DC voltage corresponding to each frequency band, positions the light filter wheel to interpose the respective light filter for three successive sector scans, and controls a Programmable Amplifier for normalizing the signals of the three bands. A single frame of film records the low frequency response through the red filter, the middle frequency response through the green filter, and the high frequency response through the blue filter. Where the primary colors are equally represented, the result is pure white. Where the proportion of the three primary colors is varied, the Spectra-Color Ultrasonogram will record that hue of the light spectrum which is specific to that proportion. Bypassing the Voltage Tuned Filter with a Coaxial Switch (Fig. 1) and recording through a clear position on the light filter wheel, permits a wide ultrasonogram to be recorded.

9MHz

7MHz 11MHz

I t--- Green

L

~- 2 M H z

- Blue

c

FIGURE 2. Drawing of frequency versus power, showing the full frequency band (top) and the three component frequency bands, with center frequencies, half-power bandwidths, and color assignments.

The Spectra-Color Ultrasonogram is recorded on Kodak High Speed Ektachrome, Tungsten type, exposed over three sector scan sweeps at a lens opening of f2.8, and processed to give a film speed of ASA 310. This emulsion type provides the proper color balance.

Calibration of the instrumentation is accom- plished by the use of a polished stainless steel target plate, positioned in the focal zone, normal to the sonic beam. Attenuation is inserted to obtain a maximum amplitude, unsaturated sig- nal. The video signal of each frequency band is displayed on an oscilloscope. The Programmable Amplif ier (Fig. 1) provides the gain adjustment to normalize the amplitude of each band as shown in Figure 3 . Normalizing the amplitude of the video signals from a surface which strongly

176 JOURNAL O F CLINICAL ULTRASOUND

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3c 3D FIGURE 3. Oscilloscope photographs of the calibration target video signal for the low (a), middle (b), and high (c) component frequency bands, and for the full band (d). The half-amplitude width corresponds to the axial resolution.

reflects all three frequency bands balances the amplification system to produce' a white image on the S.C.U.

RESULTS AND DISCUSSION

To investigate the frequency-dependent re- sponse characteristics of tissue models, targets made from different varieties of natural sponges were scanned, and both single band ultrasono- grams and Spectra-Color Ultrasonograms were recorded. Natural sponges were selected because they produce echoes similar to those of tissues (1) .

Our preliminary investigations indicate that the various color patterns observed with S.C.U. depend on a number of factors. Some of these include the signal strength, the structure of the tissue, the resolution of the system, and the shadowing of one tissue by another. Figure 4 contains three sonograms recorded from sector scans of a natural sponge target. The upper sonogram is the single frequency band echo pattern. The lower two sonograms are S.C.U.'s recorded from the same sector scan demon- strating the effects of a four decibel change in amplification. As seen in this example, the S.C.U. records the same echo pattern as the single frequency band sonogram with the addition of the color coded frequency informa- tion. Reduction of the amplification by four decibels causes a significant change in the color representation.

appear as white echo patterns in the S.C.U. This

FIGURE 4. A full band ultrasonogram of a tissue model (upper sonogram). Two Spectra-Color Ultrasonograms of the same sector scan (middle and lower sonograms) which demonstrate the effects of a 4 dB change in amplification.

For a given strength, denser structures

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FIGURE 5. The effects of shadowing in S.C.U. Sonograms are of two sponge models, one scanned through the other. Upper sonogram shows the less compact sponge (upper pattern) scanned through the more compact sponge (lower pattern). In the lower sonogram the target positions are reversed.

is illustrated by echo patterns from two sponges of different structure (Fig. 5, lower echo pattern of each sonogram). The signal strength can vary within a single sonogram by tissue attenuation as shown by the difference between the front and rear echo patterns of a single sponge (Fig. 4, middle sonogram). Time Gain Control was not employed because its effects would compensate for this desired tissue attenuation phenomenon. Figure 5 shows the change in pattern of a given tissue model at two system amplification levels. The deeper sponge echoes at the lower amplifi- cation level show a greater predominance of red, while the white echo pattern is diminished due to the loss of weaker echo responses. By varying the system amplification with the fixed attenu- ator, the investigator can change the color pattern, thus producing white echoes for tissues of given acoustic properties at various depths. This is of practical importance in clinical appli- cations (2).

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The Axial resolution of the S.C.U. differs from that of the single frequency band ultra- sonogram. The half-amplitude width of the video pulses shown in Figure 3 correspond to the axial resolution. The video pulse width, and therefore the axial resolution, of the three component frequency bands (Figs. 3a, 3b, 3c) are essentially equal. Equal resolution permits proper color registration. The half-amplitude video pulse width for the full band (Fig. 3d) is approximately one-third that of the component frequency bands, which accounts for the differ- ence in resolution between S.C.U. and single full band, ultrasonography. A metal target embedded within the sponge shown in the sonogram in Figure 4 (strong white echo centered in the upper third of S.C.U.), demonstrates proper color registration. The change in resolution is observed by comparing the dimensions of similar echoes in the wide band sonogram and the s.c .u . 's .

Initial clinical interest was focused upon the potential ability of S.C.U. t o distinguish be- tween different tissues shadowing each other. Figure 5 shows sonograms of targets made from two varieties of sponges, one of course texture and one of fine texture, oriented such that one target shadows the other. In the upper sono- gram, the sound passed through the fine sponge before reaching the coarse sponge. In the lower sonogram, the target positions were reversed. The two S.C.U.'s clearly show a different color pattern for the coarse sponge when it is scanned directly as opposed to when it is scanned through the fine sponge.

The results of this investigation have shown this method of providing color coded frequency information in B-scan ultrasonography to be workable and easily adaptable to clinical applica- tions. The interpretation of the different color patterns and their relationships to different tissue types depends on a number of factors which require further investigation.&

REFERENCES Sokollu A, and Holasek E: Model study of propaga- tion of ultrasonic waves in ocular media, Ophthalmic Ultrasound (Proceedings of the Fourth International Congress of Ultrasonics in Ophthalmology, Phila- delphia, Pa., 1964). The C.V. Mosby Company, St. Louis, 1969, pp. 88-89, Purnell EW, Sokollu A, Holasek E, and Cappaert WE: Clinical spectra-color ultrasonography. J Clin Ultra- sound 3, 1975.

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